1. Field of the Invention
This invention relates to a thermoplastic resin composition suitable for obtaining articles having self lubricating property, dimensional stability, thermal resistance and rigidity in a well-balanced combination.
2. Description of the Prior Art
Since polyolefins having an ultrahigh molecular weight, for example polyethylene having an ultrahigh molecular weight, in comparison with general-purpose polyolefins such as polystyrene, have a weak intramolecular cohesive force, a symmetrical molecular structure and a high crystallinity, they have excellent sliding property, impact strength, abrasion resistance and tensile strength, and can be used, for example, as a sliding material. However, because of their high molecular weights, they are difficult to shape, and in many cases, ultrahigh-molecular-weight polyethylene which is in widespread use is difficult to shape by general conventional methods.
Various proposals have been made to impart excellent moldability to ultrahigh-molecular-weight polyolefins without impairing their excellent properties.
For example, Japanese Laid-Open Patent Publication No. 12606/1988 discloses an injection-moldable polyolefin composition comprising 15 to 40 parts by weight of an ultrahigh-molecular-weight polyolefin having an intrinsic viscosity [.eta.], measured in decalin at 135.degree. C., of 10 to 40 dl/g and 85 to 60 parts by weight of a polyolefin having a low to a high molecular weight and an intrinsic viscosity, measured in decalin at 135.degree. C., of 0.1 to 5 dl/g. This composition is innovative in that in spite of its inclusion of an ultrahigh-molecular-weight polyolefin, it can be injection-molded, and molded articles obtained by injection-molding have the excellent slidability and moldability of the ultrahigh-molecular-weight polyolefin. However, this injection-molded article tends to shrink during molding, and it is still necessary to improve the shrinkage of the molded articles.
Furthermore, Japanese Patent Publication No. 144351/1985, for example, discloses a thermoplastic resin composition having excellent abrasion resistance and impact strength and a high limited PV value which is obtained by melt-kneading 70 to 98% by weight of a thermoplastic resin selected from the group of polyamides, polyacetals, polyesters and polycarbonates and 30 to 2% by weight of an ultrahigh-molecular-weight polyethylene powder having a specific particle diameter distribution.
Japanese Laid-Open Patent Publication No. 175069/1988 discloses an additive for improving the abrasion resistance of syntheic resins such as polyamides, polyacetals, polyesters and polycarbonates, said additive comprising 90 to 10% by weight of an ultrahigh-molecular-weight polyethylene component having an intrinsic viscosity [.mu.], measured at 135.degree. C. in decalin, of at least 6 dl/g, and 10 to 90% by weight of a polyethylene component having an intrinsic viscosity, measured at 135.degree. C., of 0.1 to 5 dl/g.